We find that the risks of immediate hypersensitivity to grass and mugwort pollens, as measured by a skin test, are positively correlated with the intensity of the first pollen season encountered in infancy. However, only indirect measures of the intensity of the pollen seasons (weather factors and vegetative growth of grasses) were available for the regression analysis performed. The risks were also dependent on the month of birth, maximal risk for both grass and mugwort pollen sensitivity being associated with birth in April-May. The results support an earlier suggestion that pollen contacts during the first 6 months of life increase the risk of pollen allergy for 20 years or more. We estimate that the elimination of early pollen contacts could have reduced the prevalence of immediate hypersensitivity to grass and mugwort pollens by approximately 20% each in the age group 0-19 years.